Wireless local area networks (WLANs) as defined e.g. in the IEEE 802.11 specifications are almost omnipresent today. The 802.11 standard makes it mandatory that all stations implement a distributed coordination function (DCF) which is a form of carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is a contention-based protocol making certain that all stations first sense the medium before transmitting. The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions. If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame. The collision avoidance aspect of the protocol pertains to the use of acknowledgements that a receiving station sends to the sending station to verify error-free reception. Although somewhat more complex, this process of accessing the medium can be seen as a meeting where everyone is polite and each person only speaks when no one else is talking. In addition, participants who understand what the person is saying nod their head in agreement.
Multihop relaying technology, where a signal is relayed through various network nodes, is a promising solution for future cellular and ad hoc wireless communications systems, such as for example WLAN or sensor networks, in order to achieve broader coverage and to mitigate wireless channels impairment without the need to use high power at the transmitter. Recently, a new concept that is being actively studied in multihop-augmented networks is multiuser cooperative diversity, where several terminals or network nodes form a kind of coalition to assist each other with the transmission of their messages. In general, cooperative relaying systems have a source node multicasting a message to a number of cooperative relays or nodes, which in turn resend a processed version to the intended destination node. The destination node combines the signal received from the relays, possibly also taking into account the source's original signal.
In S. Shankar et al., “Cooperative communication MAC (CMAC)—A New MAC protocol for Next Generation Wireless LANs”, IEEE WireComm, June 2005, and Bletsas et al., “A Simple Cooperative Diversity Method based on Network Path Selection”, IEEE Journal on Selected Areas of Communications, March 2006. (MIT), two methods are described, which are based on so-called “backoff process” which is widely used in wireless communication systems, especially in WLAN and WPAN systems like IEEE 802.11 and IEEE 802.15 as well as wired communication systems, such as IEEE 802.3 (Ethernet). The backoff process is used in communication systems for random access to a channel among a number of multiple contending stations, as indicated for example in D. Bertsekas and R. Gallager, “Data Networks”, Chapter 4, Prentice Hall, 1992. Usually it is combined with the CSMA protocol. The backoff process has been introduced to allow fair access to a channel by all participating stations and to adjust transmissions according to network congestion level in a distributed way.
Before a station starts a transmission, it selects a random backoff number, BO, within a certain contention windows (CW), say [0, CW]. A station selects the random number, BO, uniformly across the contention window so as to achieve fair access to the channel. If a station discovers after sending a packet that the transmission resulted in a collision because other stations were transmitting, the station will double its contention window to 2*CW and repeat the process. To discover a collision, a station either uses collision detection circuitry or relies on the receiver to inform it.
The backoff process may be reused for the purpose of selection of a cooperative node. In both solutions only single stage backoff process is designed. In Bletsas et al., “A Simple Cooperative Diversity Method based on Network Path Selection”, IEEE Journal on Selected Areas of Communications, March 2006. (MIT), the backoff process is suggested to be reused for selection of a best cooperative node.
FIG. 2 shows a flow diagram of a basic of operation according to the backoff process which may be used for selecting a cooperative node. Here according to its instantaneous channel condition, each cooperative node determines in step S101 a backoff number (BO) that is inversely proportional to the instantaneous channel condition (CC) with a factor of lambda. It then senses the channel and if it determines in the loop procedure of steps S102 to S105 that the channel is idle for BO slots, it then starts a transmission in step S106. Otherwise, if it determines in step S103 that the channel is busy, it defers and waits until the channel is idle again. Hence a cooperative node with the best channel condition will determine a smallest BO number and will seize the channel when the channel is idle for BO slots.
While this method is simple to implement and can be easily accepted because the backoff process is well known, it has the difficulty to choose an appropriate factor lambda. This is because the distribution of cooperative nodes in reality is unknown and is unlikely to follow a uniform distribution. Furthermore, there could be a case where all cooperative nodes have either very good channel condition or very bad channel condition. For example, if cooperative nodes have a lognormal distribution but with different mean values, the factor lambda has to be chosen to a large number so as to reduce potential collision between cooperative nodes with unknown distributions. However, the overhead will be increased as a larger lambda means a larger BO number and a larger idle time before contention.
This leads to the problem that the backoff process is targeted for fair arbitration of competing stations while it is intended to be unfair in selection of a best cooperative node. Therefore applying backoff process without knowing the distribution of cooperative nodes will not be efficient. If for example cooperative nodes have relative strong channel conditions, a lambda value of e.g. 27 may result in 95% successful selection of the best cooperative nodes with an average delay of e.g. 10 slots. Otherwise, if cooperative nodes have relative weak channel conditions, the above lambda value of 27 may result in 96% successful selection but the average delay in selection may now be 55 slots, which is substantially larger. Thus, a simple backoff-based selection procedure will not suffice to provide fast selection of the best cooperative node.